1. Field of the Invention
The present invention relates to an electroacoustic transducer based on the capacitor principle. The transducer includes an essentially rigid electrode and a flexible electrode constructed as a diaphragm, wherein in the central areas of the electrodes the distance between the diaphragm and the rigid electrode is smaller than in the border areas thereof. The transducer may also include a resonator attachment.
2. Description of the Related Art
A transducer of the above-described type is disclosed in W082/00745 A1. In this transducer, the rigid electrode has a convex shape on its side facing the diaphragm, so that the changing distance between the diaphragm and the rigid electrode is obtained from radius to radius. The rigid electrode is held only by individual hooks in the upper area of an insulating housing, wherein air can flow between the hooks into a chamber formed behind the fixed diaphragm, so that the characteristics, and especially the pickup pattern, of the microphone can be adjusted to the desired values.
Another transducer with a diaphragm and a rigid electrode is disclosed in U.S. Pat. No. 4,796,725 A. In order to be able to operate in the ultrasonic range, the diaphragm is pulled onto the convexly constructed electrode and is raised a small distance from the electrode by applying a direct voltage with superimposed alternating voltage, on the one hand, and is caused to vibrate with correspondingly high-frequency vibrations, on the other hand. Aside from the vibration modes occurring as a result, the diaphragm remains always at the same distance from the electrode or has a slightly greater distance in the center as compared to the border where the diaphragm is mounted.
A completely different transducer is constructed as a hydrophone and is disclosed in SU 1 784 111 A3. In this transducer, a rigid electrode and an electrode capable of oscillation are used, wherein the electrode capable of oscillation is composed of a dome-shaped Teflon plate whose border rests freely outside of the rigid electrode against a housing projection and whose apex rests against a screw cap, wherein by turning the screw cap the height of the Teflon dome can be changed by elastic deformation and the radiation characteristic of the hydrophone can be adjusted to the desired values.
In general, transducers based on the capacitor principle, to which the present invention relates, are composed of a rigid electrode and a flexible electrode which is held tensioned in front of and at a slight distance from the rigid electrode, wherein, when the transducer is used as a microphone, the flexible electrode is oscillated by the impinging sound waves, so that the capacity of the capacitor formed by the two electrodes is changed which, appropriately converted and amplified, completes the transducer.
The flexible electrode may be of synthetic material and be coated with an electrically conducting material, possibly gold, and the rigid electrode, depending on the type of charge application, may be of a conductive or insulating material with applied charge carriers or an electrically conductive coating or layers. The contacting can be effected in various ways and is not part of the invention.
In transducers of this type, it is possible to either apply an external voltage (true capacitor principle) or to apply charge carriers permanently on one of the electrodes (electret principle). The sensitivity of such a transducer can be influenced by changing the distance between the electrodes and/or by changing the applied voltage (or by changing the charge in the case of the electret principle). An increase of the voltage or mounting the electrodes closer together in their positions of rest lead to an increase of the sensitivity which, when used as a microphone, is to be defined as the ratio of the output voltage to the sound pressure.
The operation of these transducers is satisfactory and, compared to electrodynamic transducers in which the diaphragm is provided with a coil, these transducers have several advantages: because of the extremely small mass of the diaphragm which forms the flexible electrode, the transducers can also convert very high-frequency sound waves.
However, these transducers also have disadvantages which are primarily due to the limits of the sensitivity or the voltage, or of the charge which can be applied in the case of the electret principle, and which are due to the requirement to maintain a minimum distance between the electrodes, because the charge induces an attracting effect between the electrodes which may have the result that, particularly in the case of greater movements of the diaphragm, the diaphragm is moved so closely to the rigid electrode that the attracting forces exceed the elastic restoring forces and the diaphragm makes permanent contact with the rigid electrode over a small or large area thereof, which means, of course, that the oscillating behavior in the remaining areas becomes completely unpredictable and the transducer characteristics become undefined and useless.
Since the increase of the applied voltage has a very positive influence on the transducer characteristics because the sensitivity is increased or the output signal with otherwise the same transducer parameters becomes stronger, it is now being attempted to increase the applied voltage as much as reasonably possible, or to reduce the distance between the parallel electrodes, which, of course, has the result that there are frequently problems with the diaphragm adhering to the other electrode.
The frequency pattern of such transducers, particularly of such electret microphones, is substantially influenced by a resonator attachment and the edge of this resonator attachment. The resonator attachment extends parallel to the two electrodes and is generally composed of a foil which has a significantly greater thickness than the diaphragm and is provided with openings in order to facilitate the passage of the sound and simultaneously to influence the sound. This resonator attachment is held by a ring in the capsule of the transducer (transducer housing), wherein the height of the ring also influences the transducer characteristics.
Therefore, it is the primary object of the present invention to provide a transducer of the above-described type in which the problems discussed above are eliminated.
In accordance with the present invention, in a transducer having the conventional, approximately circular shape of the electrodes, the diaphragm is deformed into a cone shape or truncated cone shape.
This configuration according to the present invention has been found in experiments to significantly increase the capsule sensitivity of the transducer. The transducer according to the present invention also surprisingly solves the problems discussed above. The rigid electrode may be flat or curved and the diaphragm may be elastically deformed or partially plastically deformed.
In accordance with a further development of the invention, an essentially circular projection is formed in the center area of the rigid electrode. The projection may be integrally formed with the rigid electrode. The flexible electrode rests against the circular projection. For example, the flexible electrode is glued to the projection. In the case of an electret microphone, this center area is preferably free of charge carriers.
In accordance with another development of the invention, an essentially circular projection is formed at least in the center area of the resonator attachment, wherein the circular projection is directed toward the diaphragm. The circular projection may be formed integrally with the resonator attachment. Also in this case the diaphragm may be glued to the projection in order to ensure the diaphragm is not pulled toward the rigid electrode and adheres to the rigid electrode under the influence of the opposite charges.
As a result of the measure of mounting the electrodes closer together in the central area, it is in fact unexpectedly possible to significantly increase the capsule sensitivity, without having the undesired effect of the flexible electrode adhering to the rigid electrode. It has additionally completely surprisingly be found that an increase of the characteristic in the bass range occurs, so that the linearity of the transducer reaches further into the bass range as is the case in transducers according to the prior art.
In accordance with a first embodiment of the invention, the flexible electrode or diaphragm is pressed against the projection, preferably by means of a nipple, which is mounted or provided in the center of the resonator attachment and presses the flexible electrode against the projection while elastically deforming the resonator attachment.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.